In existographies, James Watt (1736-1819) (IQ:175|#217) (Cattell 1000:225) [RGM:130|1,500+] (Gottlieb 1000:110) (Murray 4000:1|T) (EP:28) (SIG:8) (CR:109) was a Scottish instrument maker turned engineer noted for number of inventions and design improvements to the functionality of the steam engine, including: separate condenser (1765), the fly-ball governor (1788), the definition of "pony power" (or horse power), all embodied in what came to be known as the Watt engine.

PT graph
In 1764, Watt, puzzled by the breakdown in the “law of mixtures” in respect to calculating the efficiency of steam engines, based on four datum points, shown by circle dots below, made the following PT graph: [12]

On this graph, he commented:

“From these elements I laid down a curve in which the abscissa [y-axis] represented the temperatures and the ordinate [x-axis] the pressure, and thereby found the law by which they were governed sufficiently near for my then purpose.”
— James Watt (c.1764), “on his pressure-temperature graph” [12]

In 1792, Davies Gilbert was claiming and or demonstrating in court some type of calculus based integration of the work or efficiency of a Hornblower engine (1781).

In 1796, John Southern, Watt's employee, invented the the "indicator", and correlative and "indicator diagram", which tracked the changes in volume of the piston, which was stimulated in some way by Gilbert's court proceedings.

Murdock
In 1781, William Murdock (1754-1839), Watt's employee, invented the sun and planet gear. [4]

Anderson
In 1757, John Anderson (1726-1796) (Ѻ), a professor of natural philosophy at Glasgow College, asked Watt to repair a model of a Newcomen engine that Anderson used in lecture demonstrations. [10]


The scaled down model of a Newcomen steam engine that Watt began to repair, at the University of Glasgow, in 1763, which led to the idea of of the separate condenser (cold body).

The scaled down model of a Newcomen steam engine that Watt began to repair, at the University of Glasgow, in 1763, which led to the idea of of the separate condenser (cold body).

Newcomen engine | Model sized
Shown below (left) is the model Newcomen steam powered atmospheric engine that James Watt tried to repair in 1763. It was his failure to coax the machine back to optimum capacity that sparked his invention of the separate condenser, held at the Hunterian Museum, University of Glasgow (Ѻ)

Condenser
In 1765, Watt commented in what seems to be autobiographical journal format that: [2]

“Two years before I was given the job of repairing a model Newcomen steam pump for the University of Glasgow and realized the great inefficiency of this engine. But compared with muscle-powered pumps, Thomas Newcomen’s engine represents technical progress. Even the first example from 1711 was able to replace a team of 500 horses that had powered a wheel to pump out a mine. In over 50 years few detailed changes have been made to the basic design. Seventy-five of these engines can now be found at mines all over England. Coal is dearly needed, because wood has become scarce. But following the seams of coal down into the earth, the mines get flooded by water. Getting that water out of the mines is vital for exploiting the only available substitute for wood fuel. But Newcomen’s pump consumes too much of the coal it unearths.”

He continues later:

“A separate condensing chamber for the steam could save much fuel! That’s it!”

This insight was the start of Watt's improved engine.

In 1765, Watt built the following first-draft laboratory sized version of his steam engine with a separate condenser, where A is the boiler, C and D are the piston and cylinder, and G and K are the "separate condenser" consisting of two pipes of thin tin plate, 10 or 12 inches long and 1/6th of an inch in diameter, which was connected to K a air pump used as a "snifting valve", the entirety of which being set in a cistern of cold water, the machine being able to lift an 18-pound weight E: [8]

The following is a retrospect rendition the basic design of the so-called "separate condenser":

The operation of which is described as follows:

“In which a represents the cylinder, and b its plug or piston made to fit air-tight. The pipe d is furnished with a stop-cόck, by means of which the elastic vapor is occasionally admitted. A similar pipe, furnished with a stop-cόck at f, passes from the other side of the cylinder, and enters the vessel g; e being the reservoir to contain water. If we now suppose the piston at the bottom of the cylinder, and steam admitted by the pipe d, its expansive force will elevate the piston, and when the air is expelled, the whole internal cavity of the tube will be filled with condensible vapor. On closing the steam-cόck, and opening that connected with the vessel g, a portion of the vapor will immediately expand itself, and coming in contact with the cold sides of the vessel, a portion of its heat must be absorbed by the water at e. A new portion of steam then descends, and is also condensed, and indeed the same process continues till the whole of the steam is drawn from the tube. A vacuum being thus formed, the pressure of the atmosphere will preponderate, and the piston rod be depressed to the bottom of the tube. On closing the stop-cόck f, a new supply of steam may be admitted by the other pipe, and after raising the piston, the process of condensation may be readily repeated.”
— Charles F. Partington (1825), note on the history of the steam engine [9]

Here, we see, in the tub of water e, the roots of the concept of the "cold body" as envisaged by Sadi Carnot (1824).

In Nov 1774, Watt announced the successful trial of his new-designed full-sized steam engine, with a separate condenser, erected at Kinneil, near Boroughstones, with an 18-inch diameter cylinder, as shown below: [8]

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An 18th century artwork entitled “First steam experiment of James Watt” (see: tea kettle anecdote) based on Watt’s cousin Marion Muirhead’s later recollected apocryphal story of how Watt, at age thirteen, became intrigued by the force of steam of a teapot to later go on to “invent the steam engine”, which of course is fiction: Denis Papin invented the steam engine (see: Papin engine, 1690), by watching his pressure release valve bubble up and down on his pressure cooker in the 1680s. [3]

First steam experiment of James Watt (18th century)

An 18th century artwork entitled “First steam experiment of James Watt” (see: tea kettle anecdote) based on Watt’s cousin Marion Muirhead’s later recollected apocryphal story of how Watt, at age thirteen, became intrigued by the force of steam of a teapot to later go on to “invent the steam engine”, which of course is fiction: Denis Papin invented the steam engine (see: Papin engine, 1690), by watching his pressure release valve bubble up and down on his pressure cooker in the 1680s. [3]

Thermodynamics
Watt’s indicator diagrams were later utilized by French engineer Émile Clapeyron to graphically analyze the work output during a Carnot cycle.

Black
The Scottish physicist and chemist Joseph Black was an existence-long friend and mentor of Watt. [1] Of note, there is a embedded legend, originating from the mis-information of John Robinson, an existographer of both Black and Watt, that is extensive in 19th century history books, that Black told Watt about his theory of latent heat, whereupon Watt perceived the idea for the "separate condenser". Watt, however, latter denied this legend. [9]

Education
Watt, as a youth, studied Euclid's Elements and Isaac Newton's natural philosophy. [3]

In 1751, Watt, age 15, had read Willem Gravesande’s Elements of Natural Philosophy twice. [10]

Quotes | On
The following are quotes on Watt:

“James Watt was equally distinguished as a natural philosopher and chemist; his inventions demonstrate his profound knowledge of those sciences, and that peculiar characteristic of genius, the union of them for practical application.”
— Humphry Davy (1824), “Address to Royal Society” (Ѻ)(Ѻ)

“James Watt who, directing the force of an original genius, early exercised in philosophic research, to the improvement of the steam engine.”
— Henry Brougham (1825), epitaph below statue (Ѻ)(Ѻ) of Watt at Westminster Abbey

“Watt started as an instrument maker and went on to be a practicing engineer. His big idea came to him while he was assisting in the laboratory of Glasgow University where the Scottish scientist Joseph Black was lecturing on heat. Watt noted the waste of heat between strokes in the model of the Newcomen engine, which he was repairing, because the walls of the cylinder had to be cooled and re-heated with each cycle. He continued to use steam at atmospheric pressure and a partial vacuum, but he provided a separate chamber for condensing the steam, connected with, but apart from, the cylinder around which he put a steam jacket to keep its walls hot. Thus, he saved three-fourths of the fuel which Newcomen had required.”
— Richard Kirby (1959), Engineering in History [6]

“Watt, in 1761 or 1762, with his friend John Robinson, who was then an undergraduate, carried out experiments with a Papin digester—an early form of pressure cooker. He used a syringe with a plunger as a makeshift cylinder and piston, and found that the pressure of steam from the digester was enough to cause the plunger to raise about fifteen pounds, a considerable weight.”
— Donald Cardwell (1971), From Watt to Clausius (pg. 42)

“In a rather tentative fashion, knowledge of the work of Watt and of Black was spread abroad by authors such as J.A. du Luc and Joao Magellan, and the anonymous author of the 1770 volume.”
— Donald Cardwell (1971), From Watt to Clausius (pg. 55) [11]

Quotes | By
The following are quotes by Watt:

“I can think of nothing else than this machine.”
— James Watt (1765), “Letter to Dr. Lind” (Ѻ), Apr 29

“I intend in many cases co employ the expansive force of steam to press on the pistons, or whatever may be used instead of them, in the same manner in which the pressure of the atmosphere is now employed in common fire engines [Newcomen engines].”
— James Watt (1769), Patent description; cited by Richard Kirby (1959) in Engineering in History (pgs. 167-68)

“In the winter of 1763-64, having occasion to repair a model of Newcomen’s engine belonging to the Natural Philosophy [John Anderson’s] class at the University of Glasgow, my mind was again directed to it. At that period, my knowledge was derived principally from Desaguliers [Natural Philosophy], and partly from Belidor [Hydraulic Architecture].”
— James Watt (c.1800), Publication [5]

“It was well known to some philosophers that the capacity [see: heat capacity] or ‘equilibrium of heat’ as we then called it, was much smaller in mercury and tin than in water.”
— James Watt (1814). “Letter to David Brewster”, May [9]

See also
● John Roebuck
● Matthew Boulton

References
1. Sproule, Anna. (2001). James Watt – Master of the Steam Engine. Woodbridge, Conn.: Blackbirch Press, Inc.
2. Kummel, Reiner. (20110. The Second Law of Economics: Energy, Entropy, and the Origins of Wealth (Watt, pg. 16-17). Springer.
3. Marsden, Ben. (2002). Watt’s Perfect Engine: Steam and the Age of Invention (pgs. 10-11) (image). Icon.
4. (a) Burke, James. (1996). The Pinball Effect (pg. 24). Back Bay Books.
(b) William Murdoch – Wikipedia.
5. Johns, Cort. (2019). The Lost Industrial Revolution (pg. 186). LuLu.
6. (a) Dickinson, Henry W. (1939). A Short History of the Steam Engine (pgs. 68-69). Cambridge University Press.
(b) Kirby, Richard; Withington, Sidney; Darling, Arthur; and Kilgour, Frederick. (1956). Engineering in History (pg. 167). Courier, 1990.

7. Somerset, Edward (Marquis of Worcester). (1825). The Century of Inventions: Original Manuscript with Historical and Explanatory Notes and a Biographical Memoir (editor: Charles F. Partington) (pgs. 118-19). Murray.

8. Thurston, Robert. (1878). A History of the Growth of the Steam-Engine (txt) (laboratory engine, pg. 89; Kinneil engine, pg. 98). Appleton and Company.
9. Cardwell, Donald S.L. (1971). From Watt to Clausius: the Rise of Thermodynamics in the Early Industrial Age (Robinson legend, pg. 41; Brewster, pg. 44). Cornell University Press.
10. Miller, David P. (2020). “James Watt and the Business of Natural Philosophy”, in: James Watt (1736-1819): Culture, Innovation, and Enlightenment (editors: Caroline Archer-Parré, Malcolm Dick) (§7:163-88; pg. 169). Oxford University Press.
11. (a) Cardwell, Donald S.L. (1971). From Watt to Clausius: the Rise of Thermodynamics in the Early Industrial Age. Cornell University Press.
(b) Du Luc, J.A. (1786). Idee sur la Meteorologie, Volumes One to Three. London.
(c) Magellan, John. (1780). “Essay on a New Theoretical Law of the Fire Element, and of the Heat of Bodies” (“Essai sur law Nouvelle Theorie du Feu Elementaire, et de la Chaleur des Corps”). London.
12. (a) Robinson, John. (1797). “Steam-Engine”, Encyclopedia of Britannica. Publisher.
(b) Robinson, John. (1818). The Articles Steam and Steam-Engines: Written for the Encyclopedia Britannica, by the Late John Robinson, with Notes and Additions by James Watt (§: Letter from Mr Watt to Dr Brewster on the History of the Steam Engine, pgs. iii; §: On Steam, pgs. 1-45; §: On the Steam Engine, pgs. 46-151; §: Appendix by Mr Watt, pgs. 152-80). Edinburgh: Murray.
(c) Robinson, John. (1822). A System of Mechanical Philosophy, Volume Two (notes: David Brewster) (quote, pg. 17). Edinburgh, 1822.

Further reading
● Muirhead, James. (1859). The Life of James Watt (English translation: Ch. XI, Denys Pain: His memoir of 1690, Section: A New Way to Obtain Very Great Motive Powers at Small Cost”, pgs. 131-42). London: John Murray.
● Smiles, Samuel. (1865). Lives of Boulton and Watt: Principally from the Original Soho Mss. Comprising Also a History of the Invention and Introduction of the Steam Engine (pg. 35). Murray.
● Law, Rodney. (1969). James Watt and the Separate Condenser. Science Museum Monographs.

External links
● James Watt – Wikipedia.